SEDIMENTARY AND METAMORPHIC ROCKS 95 
Other Organic Locks. There are other forms of 
rock which are directly or indirectly of organic origin, 
and nearly all the sedimentary strata have some ani- 
mal or plant remains. Some of the iron deposits of 
the bogs, such’as the bog iron ore (see also p. 85), are 
made possible by the pres- 
ence of organic acids pro- 
duced by the decay of plants. 
This causes a chemical 
change, resulting in the 
accumulation of iron ore. 
Flint, and the more impure 
chert, are also made by the 
agency of organisms. These 
are dense, hard layers or 
nodules of silica, some of 
which may have been formed 
by silicious animals, although 
most appear to be of later 
chemical origin. They vary | Fic, 41. 
in color through all shades, An odlite rock, showing the tiny 
but are usually dark. ee tah ad ec 
Another form of rock that is sometimes found is 
oolite (Fig. 41). This is usually a limestone, though 
occasionally an iron or even a silicious rock. It is 
made of tiny grains, like bunches of fish eggs, whence 
the name. Hach of the rounded grains, usually minute, 
